Process for preparing cyclic polyoxymethylene

ABSTRACT

Cyclic polyoxymethylene having a number-average molecular weight of 500 to 5,000 is obtained by solid state cyclization reaction of an alkalidegradative linear oxymethylene homopolymer in the presence of a medium containing a cationic catalyst. 
     The cyclization is effected at 10° to 150° C. for 1 to 500 minutes in an organic liquid medium. 
     The reaction mixture obtained by the cyclization is treated with an alkaline medium to hydrolyze and remove away unreacted linear polyoxymethylene homopolymer, and isolate the cyclic polyoxymethylene. 
     The product has a sharp molecular weight distribution and is useful as a resin improver.

FIELD OF INVENTION

The present invention relates to a novel process for preparing a cyclicpolyoxymethylene having a relatively narrow molecular weightdistribution. The polymer can be used as a useful material for plasticadditives to improve self-lubrication, wear resistance and appearance ofmolded articles, as a starting material for synthesis of segments of anew copolymer, and as an intermediate for preparation of variouscompounds.

BACKGROUND AND SUMMARY OF INVENTION

Linear oxymethylene polymers or copolymers having repeating oxymethyleneunits (--CH₂ O--) are well known. The process for preparing suchpolymers or copolymers is roughly divided into two processes, the firstbeing a polymerization process using anhydrous formaldehyde as the mainmonomer, while the second being a polymerization process using a cyclicacetal (e.g. trioxane, a cyclic trimer of formaldehyde) as the mainmonomer.

The polymerization or copolymerization of substantially anhydrousformaidehyde in the presence of an anionic or cationic catalyst has beenproposed as the first process, and a process for polymerizing orcopolymerizing a low cyclic acetal as the main monomer, such astrioxane, in the presence of a cationic catalyst has been practiced asthe second process.

Generally, a polymer or copolymer prepared by such processes has alinear structure and a broad molecular weight distribution, and has beenwidely used as materials for engineering plastics.

Apart from such linear oxymethylene polymers, the inventors have foundthat a specific macrocyclic polyoxymethylene which has a relativelysmall molecular weight distribution and a molecular weight not more than5000 can be obtained as a part of products in a cationic polymerizationof a cyclic monomer such as trioxane. A technique to isolate the cyclicpolymer was established also by the same inventors and proposed inJapanese Provisional Publication No. 55712/1990. However, in such acase, it is not economical to isolate the desired cyclic polymer in alarge amount without investment of additional polymerization equipmentbecause the desired macrocyclic polymer is produced in a small amount asa by-product during the polymerization process of trioxane or the like.

It has now been found that when a conventional linear oxymethylenepolymer is treated with a cationic catalyst, there is obtained a cyclicpolyoxymethylene polymer which is stable to alkali and has a relativelylarge ring structure (number average molecular weight being 500 to5,000) and has no terminal groups.

The present invention is related to a process for preparing a cyclicpolyoxymethylene which comprises subjecting an alkali-degradative linearpolyoxymethylene homopolymer to a solid state cyclization reaction bytreating it with a medium containing a cationic catalyst to therebyprovide a cyclic polyoxymethylene having a number average molecularweight of 500 to 5,000, and treating the resultant reaction mixture withan alkaline medium to remove the unreacted linear polyoxymethylenehomopolymer by hydrolysis to thereby isolate the desired cyclicpolyoxymethylene.

The cyclic polyoxymethylene obtained by a process of the presentinvention is a specific polyoxymethylene having a relatively large ringstructure (number average molecular weight of 500 to 5,000) and noterminal group, and is represented as the following general formula:##STR1## wherein n=17 to 170

According to test obtained by means of NMR, infrared spectrophotometry,and GPC as well as a hydrolytic cleavage test by alkali, the cyclicpolymer formed by the process of this invention has a relatively largering structure, no terminal group and a relatively narrow molecularweight distribution.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred process for preparing the cyclic polyoxymethylene of thepresent invention will now be described in detail.

The preferred raw material used in the present invention is analkali-degradative oxymethylene homopolymer. An oxymethylene comonomeris not suitable in such a case because the linear copolymer is stable toalkali and unable to be decomposed by an alkali treatment of theresulting reaction mixture containing the unreacted linear copolymer,and is unable to be separated from the cyclic polymer.

Similarly, a linear homopolymer having both terminals protected withgroups stable to alkali is also not suitable for the same reason. Anyalkali-degradative oxymethylene homopolymer may thus be employed as araw material used in the present invention. An example of one suchusable homopolymer is a commercially availabie homopolymer havingterminal groups protected with ester groups such as acetyl groupswherein the homopolymer can be cleaved by alkali.

When the homopolymer meets the above-mentioned requirements, anyoxymethylene homopolymer such as those prepared by the cationicpolymerization of trioxane or by an anionic polymerization offormaidehyde may be employed as a starting material of the presentinvention.

The process of the present invention is especially characterized in thata linear polyoxymethylene homopolymer is treated with a mediumcontaining a cationic catalyst while the inherent crystalline state ofthe homopolymer is maintained. It was quite unexpected that suchtreatment of a crystalline linear oxymethylene polymer yields a cyclicproduct having a relatively large ring structure through a cyclizationreaction. The present invention thus makes such a reaction possible forthe first time.

In order to carry out the cyclization reaction efficiently by intimatecontact of the oxymethylene homopolymer with a cationic catalyst whilethe inherent crystalline state of the homopolymer is maintained, it ispreferable to employ the homopolymer in the form of finely dividedparticles in a medium, with the more preferred form being a suspensionof minute particles.

Any medium may be employed provided it is inactive to a cationiccatalyst. Examples of usable media include aliphatic hydrocarbons suchas cyclohexane, aromatic hydrocarbons such as toluene, halogenatedhydrocarbons such as methylene chloride and mixtures thereof. The amountof the medium should be sufficient to immerse the polymer feed, with thepreferred amount being sufficient to provide a homogeneous stirring ormixing of the reactants.

In the process according to the present invention, a cationic catalystis employed for the cyclization reaction. Examples of such catalystinclude Lewis acids, that is, halogenated boron, tin, titanium,phosphorus, arsenic, antimony and the like, especially BF₃, SnCi₄,TiCi₄, PCl₅, PF₅, AsF₅, SbF₅, and complexes or salts thereof; a protonacid such as trifluoromethane sulphonic acid, perchloric acid, esters ofa protonic acid, especially esters of perchloric acid and a lower fattyalcohol such as tert.-butyl ester of perchloric acid; proton acidanhydride, especially a mixed anhydride of perchloric acid and a loweraliphatic carboxylic acid such as acetyl perchiorate; isopoly acids;heteropoly acids such as phosphorus molybdate; triethyloxoniumhexafluorophosphate, triphenylmethyl hexafluoroarsenate;acetylhexafluoroborate and the like. Among them, BF₃ or coordinationcompounds of BF₃ with an organic compound is most preferred.

Although the reaction conditions such as the treating temperature, thetreating duration, the catalyst concentration, the concentration ofpolymer feed and the like cannot absolutely be stated because they arerelated to each other, the treating temperature range is between 10° and150° C., with the preferred range being between 20° and 130° C., thetreating duration range is between 1 and 500 minutes, with the preferredrange being between 5 to 200 minutes, the catalyst concentration rangeis between 0.05 and 10% by weight of the polymer feed, with thepreferred range being between 0.1 and 5% by weight, and the polymerconcentration range in a medium is between 1 and 50% by weight, with thepreferred range being between 2 and 30% by weight.

The rate of decomposition reaction is more accelerated with increasingseverity of the reaction conditions for the cyclization reactionresulting in a poor yield of the cyclic product. Further, the milder theconditions become, the more the reaction duration is prolonged.

The addition of a deactivator for the cationic catalyst forces thereaction to completion. Examples of the deactivator include ammonia;amines such as triethylamine, tri-n-butylamine and the like; hydroxidesof alkali metals or alkali earth metals; and conventional deactivatorsfor cationic catalyst. The deactivator may be added in the original formor in a form of solution while stirring is taking place to therebyneutralize and deactivate the catalyst. In this way, the resultingreaction mixture contains the unreacted linear oxymethylene homopolymerwith the desired cyclic polyoxymethylene, which is produced from a partof the homopolymer feed by the cyclization reaction, the former beingrecovered from the resulting reaction mixture by a suitable method.

The preferable process for recovery of the cyclic product from theresulting mixture is a process comprising decomposing the unreactedlinear oxymethylene polymer by treating the mixture with an alkalinemedium which is separately provided. That is, the separation processcomprises the steps of separating the mixed polymer from the cyclizationmedium, contacting the resulting mixture with an alkaline medium, suchas water and/or an organic solvent containing an alkaline material, withthe preferred material being those soluble to the medium employed.

Although it may be possible to replace the alkaline medium by theaforesaid medium for cyclization to which the alkaline material has beenadded beforehand to hydrolyze the unreacted homopolymer, it is preferredto provide the alkaline medium separately, because the suitable mediumfor cyclization does not always suit hydrolysis. Examples of thealkaline material include strongly basic hydroxides such as alkali metalhydroxides or alkaline earth metel hydroxides; salts of a strong basewith a weak acid; and ammonia or organic bases such as amines andamidines and the like. More specifically, examples of such alkalinematerial include caustic soda, caustic potash, sodium carbonate, sodiumacetate, ammonium hydroxide, triethanolamine, triethylamine,tripropylamine, tetramethyl guanidine, and the like.

The most convenient medium used for decomposition of the unreactedlinear oxymethylene polymer is water, but primary-, secondary- andtertiary-alcohols may be employed as a part of the medium or the mainmedium. Examples of such organic alcohol include aliphatic alcoholshaving 1 to 20 carbon atoms such as methyl alcohol, ethyl alcohol,n-propyl alcohol, isopropyl alcohol, isobutyl alcohol, anyl alcohol,lauryl alcohol, and the like. Examples of the other usable alcoholinclude aromatic alcohols, preferably those having 1 to 20 carbon atomssuch as benzyl alcohol, α-phenyl ethyl alcohol, diphenylcarbitol, andthe like; cyclic aliphatic alcohols, especially those having 1 to 20carbon atoms such as 1,2-dimethylcyclopentane-1,2-dioi,1-methylcyclohexyl alcohol and the like; polyvalent alcohols, especiallythose having 1 to 20 carbon atoms such as ethylene glycol, glycerol, andthe like.

The alkaline medium thus provided has a pH of 7.0 or more with 8.0 ormore being preferred.

The preferred hydrolysis temperature range is between 90° and 220° C.,with the more preferred range being between 130° and 200 ° C.

Although the pressure depends on the temperature and the kind of mediumemployed, atmospheric pressure as well as a relatively high pressure maybe employed.

The treatment by the alkaline medium may be conducted in either theheterogeneous or the homogeneous mode, which depends upon thetemperature and the kind of medium employed. That is, when water or anaqueous medium comprising mainly water is employed, or when a relativelylow temperature is employed, the system will be a heterogeneous mode. Insuch case, the unreacted linear polyoxymethylene homopolymer which ispresent in the resulting mixed polymer can be completely decomposed bytreatment with the alkaline medium to thereby provide formaldehyde whichis dissolved in the alkaline medium and removed from the system.

When a relatively high temperature, or a medium which can dissolve allof the mixed polymers is employed, the system will be a homogeneousmode. In such case, all of the resulting mixed polymers are treated in ahomogeneous solution to thereby decompose, at first the terminal groups,and then the whole unreacted linear oxymethylene polymer in a shortperiod of time.

Formaldehyde, thus produced, is dissolved in the medium. The desiredcyclic polyoxymethylene which exists in the medium without being cleavedcan be isolated from the system by precipitating it by cooling oraddition of a nonsolvent.

Naturally, the treatment in the heterogeneous mode requires a longperiod of time to complete the decomposition, whereas the homogeneousmode requires a short period of time.

Although the amount of the alkaline medium can be varied widelydepending on the amount of the polymer feed, the kind of the alkalinematerial employed, pH of the material and the conditions of hydrolysis,it should be in a sufficient amount to immerse the mixed polymerentirely.

The alkaline medium can selectively decompose the unreacted linearpolymer while the decomposition of the cyclic polymer is depressed byneutralization of the cationic catalyst, which can decompose even thecyclic product. When acidic medium is used, it is difficult to obtainthe desired cyclic product because said acidic medium can decompose thecyclic polymer as well.

Although the duration range required for hydrolysis varies depending onthe properties of the medium employed, the hydrolysis conditions, themode of hydrolysis, the properties of polymers, it is generally betweenabout 0.2 minutes and about several hours, with the preferred rangebeing between 1 about 120 minutes.

After completion of hydrolysis, the cyclic polymer is removed from themedium and cooled or washed with a large amount of nonsolvent followedby drying, if necessary.

The cyclic polyoxymethylene thus obtained has a number average molecularweight (Mn) of 500 to 5,000 and a molecular weight distribution with asingle peak.

According to the analyses by means of NMR and infraredspectrophotometry, the cyclic polymer has no terminal group. Moreover,the cyclic polymer is stable to hydrolytic cleavage by alkali.Therefore, these results provide conclusive proof that the cyclicpolymer has a ring structure.

From the Mw/Mn ratio of the main component, the cyclic polymer of thepresent invention is supposed to have a relatively narrow and uniformmolecular weight distribution.

A cyclic polyoxymethylene having a number average molecular weight of500 to 5,000 and a large ring structure can be obtained by a relativelysimple procedure. The cyclic product is expected to be widely used as amaterial component for improving the properties of resins, and as anintermediate for syntheses of various chemicals or new polymers.

EXAMPLES

The following non-limiting examples will further illustrate the presentinvention.

Examples 1 to 3, Comparative Example 1

A ground linear polyoxymethylene homopolymer listed in Table 1 was addedto a reactor fitted with a stirrer, mixed with a specific mediumcontaining a dissolved specific cationic catalyst shown in Table 1.Cyclization was conducted at a temperature for a period of time asspecified in Table 1. After filtration to remove the reaction medium, amedium containing an alkaline material was fed to the resulting polymermixture to neutralize and deactivate the cationic catalyst whiletreating was carried out at a temperature for a period of time tothereby decompose the unreacted linear polyoxymethylene, followed bycooling, filtration to collect the precipitate of the cyclicpolyoxymethylene, and drying after washing of the precipitate withacetone.

The following characteristic value ws measured by the following methodfor the obtained polymer.

1) Molecular weight

In GPC (solvent; hexafluorotin propanol, at 40° C.), number averagemolecular weight (Mn) and weight average molecular weight (Mw) weredetermined by means of GPCLALLS (low-angle laser light scatteringdetector) and from calibration curves with polytetrahydrofuran as astandard sample.

2) Terminal Groups: by means of 1H-NOR

The precipitate was subjected to the above-described tests.

According to the spectral tests by means of 1H-NNR and infraredspectrophotometry, any indication of existence of a terminal group inthe chemical structure of the samples was not found through the infraredspectrum, except for the signal of proton at 4.97 ppm in 1H-NNR spectrumwhich shows the existence of the --CH₂ O--group.

In order to judge by comparison, the same linear oxymethylenehomopolymer was subjected to a hydrolytic cleavage test with the samealkaline medium without treatment with the cationic catalyst in advance.All of the polymer was decomposed by the alkaline medium.

The conditions of the treatment and the properties of the cyclicpolymers obtained in examples are set forth in Table 1.

The polymer feeds used in examples are as follows: A: Hydroxy-terminatedpolyoxymethylene homopolymer having a number average molecular weight ofabout 55,000 B: Acetyl-terminated polyoxymethylene homopolymer having anumber average molecular weight of about 36,000

                                      TABLE 1                                     __________________________________________________________________________                                                 Comparative                                   Example 1  Example 2  Example 3 Example 1                        Polymer Feeds (% by wt.)                                                                   A (100)    A (100)    B (100    A (100)                          __________________________________________________________________________    Cyclization Reaction                                                          Catalyst (parts by wt.)                                                                    BF.sub.3 /butyl etherate                                                                 BF.sub.3 /butyl etherate                                                                 SnCl.sub.4 (1)                                                                          --                                            (0.5)      (10.5)                                                Medium (parts by wt.)                                                                      Cyclohexane (2000)                                                                       n-Octane (1000)                                                                          Cyclohexane (2000)                                                                      --                               Temperature (°C.)                                                                   60         30         100       --                               Duration (min.)                                                                            30         200         5        --                               Decomposition of                                                              Unreacted Linear Polymer                                                      Alkaline Material                                                                          Ammonium Hydroxide                                                                       Ammonium Hydroxide                                                                       Triethylamine                                                                           Ammonium Hydroxide               (% by wt., based                                                                           (0.5)      (0.5)      (0.5)     (0.5)                            on medium)                                                                    Medium (parts by wt.)                                                                      H.sub.2 O:Methanol =                                                                     H.sub.2 O:Methanol =                                                                     H.sub.2 O:Methanol =                                                                    H.sub.2 O:Methanol =                          1:1 (5000) 1:1 (5000) 1:1 (5000)                                                                              1:1 (5000)                       Temperature (°C.)                                                                   170        190        170       170                              Duration (min.)                                                                            15          5          25        15                              Cyclic Substance obtained                                                     M.sub.n      2000       2200       2000      All polymers                     -- M.sub.2 /-- M.sub. n                                                                    1.4        1.2        1.5       were decomposed                  Terminal Group                                                                             None       None       None                                       Yield        17         20          10        0                               (% by wt., based                                                              on polymer feed)                                                              __________________________________________________________________________

We claim:
 1. A process for preparing a cyclic polyoxymethylene having anumber-average molecular weight of 500 to 5,000 wherein said processcomprises solid state cyclization reaction of an alkalidegradativelinear oxymethylene homopolymer in the presence of a medium containing acationic catalyst.
 2. A process according to claim 1, wherein saidcyclization reaction is carried out at a temperature of 10° to 150° C.for 1 to 500 minutes in an organic liquid medium.
 3. A process forpreparing a cyclic polyoxymethylene, wherein said process comprisestreating the reaction mixture obtained by the cyclization reactionaccording to claim 1 with an alkaline medium to hydrolyze the unreactedlinear polyoxymethylene homopolymer, removing the hydrolyzed homopolymerand isolating the cyclic polyoxymethylene.
 4. A process according toclaim 3, wherein said alkaline medium is a liquid having a pH of 8 ormore and comprised mainly of water, alcohol or mixtures thereof, and asmall amount of alkaline material.
 5. A process according to claim 3,wherein said treatment of unreacted oxymethylene homopolymer with analkaline medium is carried out at a temperature of 130° to 200° C. for 1to 120 minutes.
 6. A process according to claim 1, wherein said cyclicpolyoxymethylene has a main chain with a ring structure consistingessentially of oxymethylene structural units, a number average molecularweight of 500 to 5,000 a molecular weight ratio Mw/Mn molecular weight,and Mn is number average molecular of 1.0 to 2.0, wherein Mw is weightaverage molecular weight and Mn is number average molecular weight, andwherein said cyclic polyoxymethylene has substantially no terminalgroups.